Waterproofing an exterior deck so it can be tiled

I have an exterior deck that I want to tile but first I have to waterproof it because below the deck is going to be a dry storage area. The deck has been built and has a plywood surface. Now, I just need to know the best way to waterproof it prior to laying tile on it. Any suggestions would be very much appreciated. Thanks in advance. Pierre in San Rafael, Ca.

Yes I've used a product similar once, and a contractor used it on my 1907 (I wish he would have just left it alone) entrance deck.

Don't go there, it looks fine the first year then it starts to delaminate & crack at the seams... the CA sun & rain where we are (north of Santa Rosa in Sebastopol)is a bear on it.

What I'm doing is replacing the ply & finish with traditional tongue & grooved fir which I'll paint with an epoxy based ext finish, like I did for the Heritage award winning historical restoration on the AE web site. ...but these decks aren't over an interior space.